A typical prior art STAP system comprises a phased-array antenna with N transmit elements and N receive elements. The receiver antenna gain pattern can be steered in a desired direction through a beam forming process. The STAP system operates using a pulse train and coherent pulse integration. The Coherent Processing Interval (CPI) defines the duration of the pulse train. During each CPI the transmitter sends out M pulses (or signals). The time between the beginning of a pulse and the beginning of the next pulse is called a Pulse Repetition Interval (PRI). The pulses reflect from objects at different distances from the STAP system. The antenna elements then receive the reflections of the pulses. The distance to an object (or the range) may be determined by the amount of time that passes between the sending of a pulse and receiving of its reflection, referred to as time delay. The STAP system collects the reflections for each antenna element (1 through N), for each pulse (1 through M), and for each range. The data received from these reflected signals can be conceptually assembled into a three-dimensional matrix which is sometimes called “the STAP cube.”
There is a trade-off associated with selecting an optimal PRI value for the prior art STAP systems. On one hand, longer PRI minimizes range ambiguity. In particular, it is desirable to receive reflections of one pulse from all targets before sending the next pulse. If PRI is relatively short, then a reflection received after a transmitted pulse would create an ambiguity as to whether it is a reflection of this pulse or the previously transmitted pulse. Selecting a longer PRI would mitigate the effects of this ambiguity. On the other hand, for coherent pulse integrating systems, longer PRI increases Doppler shift ambiguity. The inverse of PRI is called Pulse Repetition Frequency (PRF). PRF determines the maximum unambiguous Doppler shift for a target. Targets for which the absolute value of Doppler shifts is greater than one half PRF results in aliasing in the Doppler shift domain and appear to be at some Doppler shift with an absolute value that is less of than or equal to one half PRF.
Prior art STAP systems, which require a high PRF to attain a desired maximum unambiguous Doppler shift value are, however, limited as to the maximum allowable transmit pulse duration. As the PRF is increased, less time is available to transmit the pulse and wait for the return. For high operating frequencies, fast target velocities, and large unambiguous distances, this can result in very short pulse durations. Transmitting short pulses leads to the need to transmit high peak powers so that sufficient total energy is transmitted to the target.
Prior art STAP systems use a matched filter to detect the reflected signal. The matched filter performs well in detection of the reflected signal obscured by noise as long as the reflected signal matches the transmitted signal temporally, that is, as long as the reflected signal has not been Doppler shifted with respect to the transmitted signal. To the extent that the reflected signal has been Doppler shifted relative to the transmitted signal, the detection sensitivity of the matched filter degrades. If the Doppler shift is large enough, the detection sensitivity of the matched filter will be insufficient and an additional matched filter will be required, matching to the Doppler shifted version of the transmitted signal. The need of multiple matched filters in the prior art STAP systems is costly as it requires multiple subsequent, computationally intensive STAP detection system components.
There are two general classes of signals that can be characterized for detection through a matched filter: Doppler fragile and Doppler tolerant signals. Introducing a Doppler shift in Doppler fragile signals results in quickly degrading detection sensitivity through a matched filter. Doppler fragile signals include pseudorandom number (PN) coded signals, frequency stepped COSTAS signals, and in general most long arbitrarily modulated signals. Introducing a Doppler shift in Doppler tolerant signals results in continued sufficient detection sensitivity through a matched filter for most Doppler shifts of interest. Doppler tolerant signals include signals of very short duration and linear FM chirped signals. In terms of system performance, Doppler fragile signals can be characterized as providing high Doppler shift resolution, low probability of intercept in adversarial conditions, good performance in the presence of multiple coexisting and co-operating systems, and difficult to counter with electronic jamming. Doppler tolerant signals often are characterized as easier to generate, transmit and receive, poor Doppler shift resolution, harder to conceal from undesired receivers, and easier to counter with electronic jamming.
Because of the undesirability of multiple matched filters in the prior art STAP systems, they are typically designed to utilize Doppler tolerant signals. The inability of the prior art STAP systems to effectively process Doppler fragile signals limits the types of signals that can be used by such systems to only a few. This makes it easy for an enemy to detect the transmitted signals and use Electronic Countermeasures (ECM) to jam them.
After receiving all signals in the CPI, which comprise the STAP cube, and constructing the STAP cube, prior art STAP systems coherently process all of the received signals across all of the antenna elements at all time delay values. This coherent processing is the equivalent of a two dimensional Fourier transformation at each range value and it effectively transforms the three dimensions of the STAP cube to angle, Doppler shift (or velocity), and range time delay (or range) and a reflected signal amplitude for each three dimensional coordinate within the STAP cube. For any given angle in the STAP cube, the time delay-Doppler shift plane is equivalent to a cross-ambiguity function of the transmitted and received signals for that look angle, over the unambiguous range and Doppler extent of the given PRI. The cross-ambiguity function of a transmitted and received signal is defined according to the following equation:Ars=∫r(t+τ/2)s*(t+τ/2)exp[j2πνt]dt, 
where:                s(t) is the transmitted signal,        r(t) is the received signal,        τ is delay time, and        ν is Doppler shift.        
The prior art STAP system has to “null” out interference. The interference comes from many different sources. One type of interference is noise that is present due to the receiver thermal noise, random terrestrial natural and man made emissions, and cosmic background radiation. Another type of interference is clutter which is the result of reflections of the transmitted signals from stationary natural or manmade objects like land, bodies of water, trees, hills and buildings. When the STAP system is in motion, such as in an airborne platform, clutter manifests with Doppler shifts that are a predictable function of look angle, relative to the STAP platform motion. In particular, signals that are sent in the direction coinciding with the movement of the STAP system have a positive Doppler shift when they reflect from clutter. Likewise, signals that are sent in the direction opposite to the movement of the STAP system have a negative Doppler shift when they reflect from clutter. Another type of interferences is signals from ECM, which is a deliberate interference intended to prevent reception of reflected signals at certain frequencies.
To detect targets in real time, interference, should preferably be identified and nulled out during the CPI. The prior art STAP systems have to perform a complex and processor intensive calculations. As a part of such calculations, prior art STAP systems collect the data which comprises the STAP cube and coherently process across N pulses and M antenna elements. To achieve the nulling of the interference, this coherent process is modified by the multiplication of the inverted covariance matrix of the data in the STAP cube. The covariance matrix represents the interference present during detection. Prior art STAP systems perform real time adaptive covariance matrix generation as well as covariance matrix generation using prior knowledge and databases that correlate to the operating environment. Generation of this covariance matrix and its inverse is performed using the three dimensional STAP cube data, and it is a computationally costly process.
After the interference has been nulled out, the STAP system can identify moving targets by comparing the remaining values in the processed angle-Doppler shift-time delay STAP cube to a predetermined threshold.
For real time target detection and tracking, all calculations on a single set of received signals have to be performed during one CPI, before the next set of reflections is received. The processing of the prior art STAP systems comprises processor-intensive three-dimensional matrix computations. Some prior art STAP systems attempt to optimize these matrix computations. However, coherent processing and interference nulling in three dimensions remain to be the tasks of STAP that require significant processing resources, which severely limit the practicality, applicability and cost effectiveness of prior art STAP systems.
As mentioned above, due to the matched filter intolerance to Doppler fragile signals, the prior art STAP systems are typically limited to transmit Doppler tolerant signals. Using Doppler tolerant signals precludes using pseudorandom number (PN) coded signals, frequency stepped COSTAS signals, and in general most long arbitrary modulation type signals, which is desirable for improving range and Doppler shift resolution, target imaging, operational stealth, and defeating ECM.
The prior art STAP systems are further limited by the power constraints. In particular, because they are typically pulse train systems transmitting short pulses, they require high peak transmit power to get sufficient total energy transmitted out to the target. Transmit device power and thermal constraints limit the extent to which this peak power can be practically increased. Therefore the prior art STAP systems have a maximum practical range limitation as a direct result of their short pulse duration.
Accordingly, there is presently a need to provide systems and methods for detection and discrimination of targets in the presence of interference that will overcome the limitations and deficiencies of the prior art STAP systems.